A variety of track driven vehicles have been around for many years. Tracked vehicles vary from 100 ton military tanks and bull-dozers to 300 pound snowmobiles. Track types vary from segmented steel tracks to one piece molded rubber tracks.
One of the major design challenges with all types of tracks and vehicles is to find the most efficient way to transfer the torque of the drive mechanism to the track with minimum power loss. There are many torque transmission systems. The three most common torque transmission systems are an external drive, a friction drive and an internal drive. External drives include a sprocket with a fixed number of teeth around the circumference that drives against a rigid member attached to the track. The sprocket teeth protrude through the track to a point where the rigid members can not slip back under a heavy load. Friction drives include a wheel attached to the drive axle and drive against the inside surface of a track. The outside of the wheel and the inside of the track are typically made of resilient material such as rubber or other composites. The track tension must be extremely tight to prevent slippage. The track tension also results in power loss. Internal drive systems, also known as involute drives, have a track with drive lugs attached to the inside surface of the track. The drive lugs may be molded to the inside surface of a rubber track. The drive sprocket is made by attaching rigid drive teeth to a rigid radius wheel. The sprocket teeth drive against the internal drive lugs on the track.
Internal drive systems are generally considered the most efficient drive for tracks made of elastomeric material such as rubber track with drive lugs which are properly matched to drive sprockets are properly matched. They are properly matched when the pitch diameter of the sprocket matches the pitch line of the track. Another way of determining whether they are properly matched is when the pitch diameter of the sprocket causes the drive teeth to match perfectly with the center to center distance between the track drive lugs. In practice, proper matching is difficult to achieve since an elastomeric or rubber is resilient and can stretch or compress depending on a number of factors. One of the more common factors that causes changes in the pitch length is the variation in the load applied to a track during operation of the multi-surface vehicle. The load on the track and on the internal lugs will be higher when the vehicle is pulling a load, such as a heavy log, as compared to the load on the track applied to merely move the vehicle over terrain. Another major cause of changes in pitch length occurs when a rubber tracked vehicle is turned. The tracks are loaded differently when turning. The outside track on a turn will typically be loaded to a higher degree when compared to an inside track on a turn. The pitch length of the track varies with the variations in the load applied to the track.
Variations in the pitch length of the track results in a mismatch between the pitch length of the track and the pitch diameter of the sprocket. When using a sprocket having rigid drive teeth, the change in the pitch length along the track causes the sprocket teeth to “scrub in” or “scrub out” or both. In other words, the rigid tooth is rubbing between the individual drive lugs on the internal surface of the flat belt. This causes a loss in efficiency. Scrubbing in or out can result in extreme power loss and excessive wear on the track drive lugs and sprocket teeth.
In the past, fixed sprocket teeth have been replaced with sleeve drive portions. The sleeve drive portions may be fixed sleeves with very low friction so that the power loss from “scrub in” or “scrub out” will be minimized. An alternative design is to make the drive element with rotatable sleeves which allows the drive element to roll over a portion of the drive lug so that power loss and wear is minimized.
The use of sleeves, whether fixed and made of a low friction material or whether rotatable, many times will reduce the power loss due to the “scrubbing action” but generally gives rise to another problem. This other problem is due to the fact that current drive lug designs incorporate the design starting point related to sprockets for metal tracks. In other words, the designs have their roots in sprocket designs which are thought to be related to drive lugs on an elastomeric track. The drive lugs are generally designed as though they are teeth that must engage with openings in a track. Each drive lug is a tooth with a trapezoidally shaped portion having a trapezoidal cross section. The drive lug has angled walls. The elastomeric belt can be thought of a plurality of drive lugs attached to a flat elastomeric belt. The angled walls of the drive lug form an angle with respect to the track. The angle between the walls of the drive lug and the track are set so that the drive lug will engage an opening or mating portion in the driver and will guide itself into the opening. A problem occurs, however, in designs employing a sleeve in the drive sprocket. When the drive sprocket is driving the drive lug of the belt, the angle between the sleeve and the wall of the drive lug at the time the belt is wrapped about the sprocket and in a driven position, the sleeve appears to being presented with a slight incline to “climb”. A belt with such drive lugs may have a tendency to dislodge itself or jump out from the drive sprocket. Generally, the approach to fixing this problem is to wrap more of the sprocket with the drive belt. For example, if belt is covered along about 120 degrees of the circumference (this is commonly called the amount of wrap around the drive sprocket) and the belt is dislodging or jumping from the drive sprocket, one solution is to design the machine so that the amount of wrap around the drive sprocket is more than 120 degrees. For example, the amount of wrap may be increased to 150 degrees. Increasing the amount of wrap increases the amount of power needed to drive the belt. Another solution is to keep the amount of wrap the same but to increase the tension placed on the belt which tends to keep the belt from dislodging. Still another solution is to both increase the tension and increase the amount of wrap. Each of these solutions increases the amount of friction needed to drive the belt and increases the amount of power needed to drive the belt. In addition, the angle between the drive sprocket portion, such as a sleeve and the angled wall of the drive lug still presents an incline that the sleeve can “roll up” to dislodge. Thus, the belt still has the opportunity to “roll up” an incline to dislodge.
There is a need for a drive belt having lugs which are designed to resist dislodging or jumping off the track. There is also a need for a belt which uses less power when being driven and which uses a minimal amount of wrap around the circumference of the drive sprocket. If a belt required a minimal amount of wrap, the design possibilities would open up immensely. In addition, if the belt tension did not have to be tightened to make sure the belt stayed on the drive sprocket, the amount of power needed would be reduced which would provide for a much more efficient machine capable of moving loads with a reduced size powerplant. In addition, there is a need for a lower maintenance vehicle not prone to derailing the track. There is also a need for a sprocket which will accommodate the changes in the pitch line of an elastomeric flat track. In addition, there is a need for a sprocket and track with drive lugs which will either not “scrub” between the driving lugs or minimize “scrubbing” between the driving lugs. There is also a need for a sprocket which is self cleaning and which removes debris from the sprocket area to minimize problems associated with debris build up changing the pitch relationship between the sprocket and the flat track.